The hurdle of creating successful genetically engineered plants in major crop varieties is now being overcome sequentially on a plant by plant basis. While plant genetic engineering has been successfully demonstrated in several model plant species, most notably tobacco, carrot and petunia, these species are not considered to be economically important plant species for agricultural purposes. Researchers have therefore directed their efforts toward the genetic engineering of commercially important crop plants so that they may be improved through the use of genetic engineering.
The term "genetic engineering," as used herein, is meant to describe the manipulation of the genome of a plant, typically by the introduction of a foreign gene into the plant, or the modification of the genes of the plant, to increase or decrease the synthesis of gene products in the plant. Typically, genes are introduced into one or more plant cells which can be cultured into whole, sexually competent, viable plants which may be totally transformed or which may be chimeric, that is having some tissues transformed and some not. These plants can be self-pollinated or cross-pollinated with other plants of the same or compatible species so that the foreign gene or genes, carried in the germ line, can be inserted into or bred into agriculturally useful plant varieties.
Current strategies directed toward the genetic engineering of plant lines typically involve two complementary processes. The first process involves the genetic transformation of one or more plant cells of a specifically characterized type. The term "transformation" as used herein, means that a foreign gene, typically a chimeric gene construct, is introduced into the genome of the individual plant cells, typically through the aid of a vector, which is integrated into the genome of the plant. The second process then involves the regeneration of the transformed plant cells into whole sexually competent plants. Neither the transformation nor regeneration process need be 100% successful, but must have a reasonable degree of reliability and reproducibility so that a reasonable percentage of the cells can be transformed and regenerated into whole plants.
Although successful transformation and regeneration techniques have been demonstrated in model plant species Barton et al., Cell 32:1033 (1983), wherein the transformation and regeneration of tobacco plants was reported, similar results with cotton have only been achieved relatively recently. Umbeck et al. Bio/Technology, 5:3, pp. 263.gtoreq.266 (1987); Firoozabady et al., Plant Mol. Bio., 10, pp. 105-116 (1987).
Successful transformation and regeneration of genetically engineered cotton plants has the potential to be of significant value to this agriculturally important crop. One of the most important benefits potentially achievable from genetically engineering cotton plants is the alteration and modification of cotton fiber quantity and quality. Cotton fibers develop from epidermal cell layers of the ovule in the cotton plant. The single epidermal cells elongate to become fiber cells which synthesize an abundance of cellulose which is deposited in the form of a secondary wall structure in the cell. In the cotton fiber cell, cellulose is produced in a pathway leading from UDP-glucose by a number of enzymes including cellulose synthase. The quality of the cotton fiber is dependent on such factors as the extent of elongation and degree of secondary wall deposition. It is assumed that a number of genes as well as environmental factors regulate the physical characteristics of the fiber, such as length, thickness and micronaire value. However, the genes responsible for cellulose synthesis and fiber development in cotton plants are heretofore entirely uncharacterized at a molecular level.
The domesticated plants known as cotton are actually of several species. For example, the cultivars of Coker 312, PD3, Naked Seed are varieties of Gossypium hirsutum, while the cultivars Pima (S6) and Sea Island (Barbados) are varieties of G. barbadense. Different species of cotton are grown in different geographical locations and have different fiber qualities. Previously, the transfer of traits from species to species has proven difficult because of the incompatibility of the germplasms of the different species and the resultant hybrid instability. It is a goal of researchers in the cotton field to be able to obtain desired fiber characteristics in cultivars of high yield. For example, a cotton cultivar adapted to a specific geographic region may yield long, fine fibers with poor or moderate yield, while a second cotton cultivar at a different location may have excellent yield of fibers with shorter coarse fibers. It would be advantageous to be able to combine the characteristics of each of the fibers of the plants by genetically transforming one plant with the genes directed to the fiber characteristics of a second, different plant.